Polymer compounds are frequently used to increase the viscosity of drilling fluids, fertilizers, completion fluids, etc. to improve the fluids' ability to carry solids, prevent fluid loss, etc. Further, situations arise when it is desired to increase the viscosity of an aqueous cleaning fluid so that it releases more slowly from a hard surface that is to be cleaned of grease, oil or other debris.
Often, problems arise when attempting to viscosify fluids such as brines that have high concentrations of dissolved salts, such as CaBr2, CaCl2, NaBr, ZnBr2 and the like. When polymer is added directly to these brines, some of the difficulties encountered include, but are not necessarily limited to: (1) failure of the polymer to yield or viscosify in the aqueous environment; (2) formation of “fish-eyes”; (3) formation of a distinct gel phase; (4) slow rate of viscosification or hydration of the polymer; and (5) incomplete polymer hydration.
Failure of the polymer to hydrate is a more extreme problem when the polymer used to viscosify the aqueous fluid does not interact with the target fluid. The desired effect would be for the polymer to absorb water or “hydrate” such that it eventually becomes soluble in the aqueous medium and imparts a higher viscosity to the brine as compared with the brine without the polymer. This situation arises when there are high concentrations of salts dissolved within the aqueous fluid such as is the case with completion and workover brines. In these aqueous brines there is but a limited amount of free water available to hydrate the polymer. In addition, the small amount of water that is present in the brine can be chemically interacting with the dissolved salts and further limiting its activity. The net result is that the chemical potential differential is very low if not non-existent and hydration of the polymer does not occur.
When polymer is added to brine as a dry powder, lumps may be created that are referred to in the industry as “fish-eyes”. A fish-eye is a term that is used to describe the small polymer particles having a hydrated outer shell with a dry core. They may be prevalent in systems where there is inadequate mixing and therefore, low shear forces. Note that in this discussion particles are used to describe discrete groupings of polymer. Thus, a particle, in actuality, is a mass of tangled polymer chains that are wrapped around each other and held in close contact due to hydrogen bonding. Many polymers, on a molecular level, will form hydrogen bonds to each other creating a tight mass that does not become dispersed until the shearing forces exceed these bonding forces. Nevertheless, rapid hydration will take place at the interface between the polymer droplet and the aqueous medium. This surface hydration leads to the formation of a tight gel-like surface that limits the further diffusion of water into the core of the polymer mass, leaving interior polymer chains unhydrated. The fluid will then have different sized gel particles that do not dissolve. Dissolving is a second step in the viscosification process where the polymers that have hydrated, begin to untangle and become discrete entities unto themselves in a brine fluid. The size and number of polymer particles relates inversely to the level of agitation in the mixing vessel.
Fish-eye formation is not a good thing. Because the hydration is incomplete, the use of expensive polymer is very inefficient, as more polymer must be used to achieve the desired viscosity. In addition, these partially reacted polymer gel particles can be damaging to the producing formation of an oil or gas well. These unhydrated particles can flow into a producing formation and cause plugging. Often their successful removal is difficult if not impossible. This plugging, which can result in formation damage, may likely reduce the hydrocarbon production from the reservoir.
The polymer gels may also plug filters, limiting their on-stream time and making them difficult to clean and bring back on line. The filter blinding difficulty leads to increased downtime on a rig or production platform. Downtime on a rig relates directly to increased costs. In certain brine environments, the hydration proceeds beyond the fish-eye stage, but the brine and the gelled polymer remain as separate phases. Some of the brine hydrates the polymer, but the hydrated polymer remains as a separate phase from the brine because the individual polymer chains are still entangled. This problem is known to occur in heavy brines when the mixing capabilities are less than adequate. In both situations, inadequate mixing limits the homogeneity of the final viscosified brine. By not hydrating the polymer completely, more polymer is required to attain the desired viscosity leading to higher polymer costs.
To a certain degree, the problems with polymers like hydroxyethylcellulose (HEC), carboxymethylcellulose (CMC), hydroxyethylcarboxymethylcellulose (CMHEC), and the like may be reduced with intense mixing during addition and further improved by increasing the temperature. However, excellent mixing of polymer and brine is not always possible and seasonal variations in temperature can also lead to poor hydration of the polymer. Even with good mixing, hydration can be slow and the fish-eyes still form, albeit smaller in size. The very small fish-eyes may be much more damaging to a producing formation because they have the potential to migrate further into the formation before becoming lodged in reservoir pore throats. At this point their removal is difficult and the very effort made to remove them may lead to even more reservoir formation damage.
The aforementioned difficulties have been recognized and steps have been taken to minimize the problems by pre-hydrating brines in a controlled environment where near-impermeable gels are avoided. Thus, products like HEC powders are pre-mixed as a liquid dispersion in a non-polar environment so that the clumping and fish-eye problems caused by hydrogen bonding are reduced. U.S. Pat. Nos. 4,330,414; 4,392,964; 4,427,556; 4,435,217 and 4,496,468 describe methods of premixing of HEC polymer prior to its addition to an aqueous brine fluid.
U.S. Pat. No. 4,330,414 relates to a hydrophilic polymer composition for use in thickening aqueous mediums comprising HEC and a water miscible polar organic liquid which acts as solvating agent for the hydroxyethyl cellulose, where the solvating agent is a type which forms a semi-solid to viscous mixture with the hydroxyethyl cellulose under certain conditions. The polymeric composition alone, or in admixture with a diluting agent which is a non-solvating agent for the hydroxyethyl cellulose, can be added to aqueous mediums, particularly heavy brines, to produce well servicing fluids.
A polymeric composition useful in the thickening of aqueous brines comprising HEC, a water miscible organic liquid and an aqueous liquid is described in U.S. Pat. No. 4,392,964.
U.S. Pat. No. 4,435,217 concerns improving the thermal aging stability of liquid suspensions of hydrophilic, water-soluble polymers in an organic liquid by adding to the suspension a liquid fatty acid. Superior liquid polymer compositions having improved resistance to settling (syneresis), thermal aging characteristics, viscosity, and dispersibility in aqueous liquids comprise from about 40% to about 55% of an organic liquid, from about 35% to about 55% of a hydrophilic, watersoluble polymer, from about 2.0% to about 4.5% of an organophilic clay suspending agent, from about 0% to about 2.0% of a dispersant for the organophilic clay, from about 0.5% to about 2.5% of a non-ionic surfactant, and from about 1.25% to about 5.0% of a liquid fatty acid.
Processes are disclosed for activating HEC in U.S. Pat. No. 4,435,564 such that the HEC will disperse and hydrate in compatible heavy brines having a density greater than about 13.5 pounds per gallon (1.62 kg/liter) containing one or more salts including one or more of calcium chloride, calcium bromide, zinc chloride, and zinc bromide, at ambient temperatures such that the HEC will be at least 80% hydrated within one hour. The activation process comprises admixing the HEC with an organic liquid which has no appreciable swelling effect on the HEC and an amine activator. The invention in the '564 patent also provides activated HEC compositions and a process of increasing the viscosity of a heavy brine utilizing these compositions.
U.S. Pat. No. 4,439,333 involves polymeric compositions for, and methods of, increasing the viscosity of aqueous brine well servicing fluids by adding to the brine a composition containing HEC, a solid organic activating agent for the HEC, and a solvent for the activating agent which also functions as a suspending medium or carrier for the HEC.
A method of producing a well servicing fluid containing zinc bromide in which an activated HEC is either admixed with a zinc bromide solution containing above about 30% by weight zinc bromide, or, in the alternative, is admixed with a non-zinc bromide containing solution to produce a viscosified solution which is then admixed with a zinc bromide containing solution is described in U.S. Pat. No. 4,476,032.
U.S. Pat. No. 4,496,468 concerns a method of activating HEC such that the HEC will hydrate in heavy brines having a density greater than about 13.5 pounds per gallon (1.62 kg/l) at ambient temperature. The activated HEC compositions so produced, and viscous well servicing compositions wherein an oleaginous liquid and a compatibilizing agent are admixed to form a viscous slurry, admixing therewith an aqueous solution of an inorganic salt which has an exothermic heat of solution, and thereafter admixing HEC therewith to form a viscous pourable composition are described.
Alkali and alkaline earth metal and zinc halide brines are also known to be viscosified with compositions incorporating a viscosity inducing hydrophilic polymer, mineral oil, oil soluble non-ionic surfactants, polar solvents, and diluent.
In spite of these improvements, concerns still exist. Pre-hydration of polymer, once started, is not easily stopped. Thus, a dilemma arises: in order to have rapid hydration or viscosification on an oil exploration rig e.g., either a powerful pre-hydrator must be added or a large amount of polymer must be added. This addition is not only expensive, but also it initiates the hydration of the polymer in the container or vessel that cannot be easily stopped. If the container used to inventory the polymer is not used promptly, then hydration may go to completion or to a point where the material cannot be removed from the container at all. This is not desirable because the polymer in the container must be in a liquid state where pourability is maintained. Otherwise, the labor costs associated with getting the polymer into the brine system will be excessive. Further, when the polymer hydration is near completion, it often has a rigid gel structure that prevents the homogeneous viscosification of the brine. Another problem that parallels that of the fish-eyes is now encountered. Solutions to these problems may involve: (1) putting less HEC or other polymer into the container, or (2) adding more of a non-reactive diluent to limit the rate of polymer hydration. These supposed remedies have their downside as well. By inhibiting the rate of pre-hydration, the rate of hydration in the brine is reduced, increasing the amount of costly rig time. Also these measures limit the amount of polymer that can be put into the container causing more containers being required at the rig location to achieve the desired effect. This means that more of the associated chemicals in the admixture per unit weight of polymer must be increased to achieve the desired viscosity effect. In addition, the labor, container and freight costs associated with this remedy will increase. Also, when the polymer concentration added from the container is low, unacceptable lowering of the brine fluid density occurs. Now additional amounts of the expensive high density brine must be added to bring the brine back to the desired density. The higher the density difference between the polymer admixture and the brine, the more serious this problem becomes. For example for high density zinc brines at the upper limit of their density, compensation with additional brine cannot be achieved without adding special formulations.
With all of these recognized issues, still another less apparent problem exists. Until the some of the approaches discussed above, fish-eyes were quite visible to the rig personnel because they were relatively large and blinded filters and shaker screens. They surmised that these larger “gel-balls” would also cause damage to the producing formations. The improvements did not necessarily cause them to disappear. Instead, they became smaller and less noticeable. While the problems associated with screens and filters were reduced, the potential for formation damage increased. These very small fish-eyes now could proceed further into the formation before causing the plugging of pore throats. This interior damage is much more difficult to reverse and much more expensive, as it reduces the well production and revenues.